Belgian Train Questions

The first being that the Belgian Rail pass includes 10 one way journeys to any place in Belgium and can be shared among multiple people (3 in this case)

Yes, but not traveling together at the same time. If I get this right, 3 of you were staying in Brussels sharing the pass, and one day one person took the pass to go to Bruges and back one day, and the next day another person took the pass and went Ghent and back, and another day the third person took the pass to go to Antwerp and back. That is allowed, as the pass is not nominative to a single person. But if you all want to travel together, you will need 3 passes. at least that is my interpretation of the google translate version of the French terms. You appear to be able to make stopovers as long as it is on the shortest point of your origin and destination stations, but I think you need to buy supplements for the Dutch segments, as well as a supplement to travel to and from Brussels airport. Here are the terms in French. Plug it into Google translate and see what you think. They can be purchased in Belgium, at a window or a kiosk.
http://www.belgianrail.be/en/travel-tickets/passes-cards/~/media/0B0F94637A874FD6AF4FEEE752AC7CEC.pdf

We used one of those Belgium pass last year. My husband and I shared one pass.

The second bullet point under "Comment l'utiliser" means:

"Your pass is not restricted to a named person and can be used by other travellers. In this case, fill in one line per user and per journey".

I think that this does mean that you can share the pass between multiple people on the same journey as long as you tick off one space on the pass for each user. I've done exactly this in the past.

Use the website of the NMBS/SNCB for information on group tickets and passes.
You can indeed break the journey as long as you go on travelling in the right direction. Consider having lunch in Mechelen (Malines). A nice historical city which gets almost unnoticed by those travelling between Brussel and Antwerpen.

Yes, two people can use the same 10-pass together. You can walk up and buy it anytime, but you need an attended station with humans on duty. Long lines are common at big stations, especially at rush hour and school times.

Brussels to Amsterdam is an INTERNATIONAL journey. The Belgian rail pass would only be valid to the last station before the border. That's what you'd write in the destination space. You'd have to buy single tickets for the rest of the way.

The first being that the Belgian Rail pass includes 10 one way
journeys to any place in Belgium and can be shared among multiple
people (3 in this case). Does the Belgian Rail Pass have to be
purchased in advance or can you purchase it on the day you take the
first train journey?

We bought the 10-journey pass in Brussels the day of the first of our 5 rail journeys, and yes, my husband and I were able to share it. We had to fill out a line for EACH of us for EACH train journey so with three of you, you'd each get 3 one-way journeys (= 9) out of a single pass with one left over.

Thanks for the informative replies everyone! I always feel better when someone else can confirm what I think I understand!

TonfromLeiden thank you for the suggestion of Mechelen (Malines). I will check it out!

Tim, thanks for the information that I could use the pass to go as far as Antwerp and then purchase individual tickets the rest of the way. That actually hadn't occurred to me to do it that way before, but I think it is going to work out better than my original plans!

If you do hâve lunch in Mechelen/Malines, try the Witten Vos restaurant. My father and I lovéd it so much that we ate there 3 times in one day!